The California Democrat had been coy for days over whether she would support the plan being hammered out in closed-door negotiations to fund the government past a Thursday-at-midnight deadline.

After the bill was unveiled late Tuesday, Pelosi at last staked her position – and demanded that two provisions, dealing with financial regulation and campaign finance, be taken out.

"These provisions are destructive to middle class families and to the practice of our democracy," Pelosi said. "We must get them out of the omnibus package."

She later told reporters her members are “very, very concerned” about them.

The disagreement ups the pressure on lawmakers to at least pass a stopgap spending measure to keep the government humming past Thursday, and buy time for Congress to agree on a longer-term plan.

Though Democrats are in the minority in the House, Speaker John Boehner likely needs their support to pull the $1.1 trillion spending bill over the finish line. Dozens of conservative Republicans stand to defect on the vote over myriad concerns, including complaints that the bill does not try to defund President Obama’s immigration executive actions.

The provisions that rankled Pelosi were one changing the 2010 Dodd-Frank regulation of complex financial instruments known as derivatives and one sharply hiking limits on donations to political parties.

A spokesman for Boehner, R-Ohio, said the provisions were part of bipartisan trade-offs and will stay in the bill.

Nonetheless, other top Democrats were lining up to vote against the measure.

"I'm not going to support it. I've already found lots of provisions that are against the public interest," said Rep. Chris Van Hollen, D-Md. "I find it surprising that some people are threatening to shut down the government in order to extract big benefits for big banks at the expense of consumers and taxpayers."

Sen. Elizabeth Warren, D-Mass., a favorite of progressives, also blasted the package and backed Pelosi’s demands to take out the controversial provisions. She also would not say whether she would try and hold up the bill on the Senate side, should it pass the House.

The 1,603-page measure was unveiled late Tuesday and its provisions will be scrutinized in advance of a House vote set for Thursday.

The bill finances the day-to-day operations of every Cabinet department except Homeland Security through Sept. 30, 2015. However, the plan would only fund the Department of Homeland Security through Feb. 27, 2015. That is a move by House GOP leaders to tee up a debate in early 2015 over the president's recent executive action that could suspend deportation for as many as 5 million illegal immigrants.

Some conservatives nevertheless want to wage that battle now, and use the current spending bill as leverage. Though the House voted last week against Obama's immigration plan, these lawmakers want to do more.

"I feel like the leadership is asking us to punt on first down," Rep. Mo Brooks, R-Ala., said Wednesday.

Rep. Matt Salmon, R-Ariz., said he thinks over 50 lawmakers, including himself, are opposed to the bill.

Boehner defended the package and stressed that they'd revisit the immigration issue next year. "Without a threat of a government shutdown, this sets up a direct challenge to the president's unilateral actions on immigration when we have new majorities in both chambers of Congress," he said Wednesday.

The opposition by conservatives and liberals leaves House GOP leaders squeezed on both sides, forced to either make changes or carefully align a bipartisan majority to pass it, if possible.

The breakdown in the House is currently 234 Republicans and 201 Democrats -- Boehner can only lose 17 of his own before needing Democrats to pass legislation. Given the complaints aired Wednesday, it's possible that Democrats could be jumping off in greater numbers than anticipated.

One source close to the vote-whipping operation told Fox News, "we'll get there," when asked about the attrition of Democratic support.

Another source close to the whip operation suggested that going into the vote on Thursday, the GOP may have a number of members who want to vote against the bill, but will be standing by to vote "yes" in case their ballots are needed.

GOP leaders distributed a lengthy roster citing wins on spending cuts and policy "riders" and claimed significant support among the rank and file.

"I think we won on policy, the budget numbers are lower than I ever thought it would be," said Rep. Richard Hudson, R-N.C.

The campaign finance reforms would sharply increase limits on the amount that an individual may contribute to various national political party accounts each year. Those limits would rise from $32,400 to $324,000 for donations to finance parties' national conventions, election recounts and headquarters buildings. That means individuals could give $648,000 in a two-year campaign cycle, with a married couple capped at almost $1.3 million for an election

The changes to Dodd-Frank would weaken new regulations that require banks to set up separate affiliates to deal in the more exotic and riskier forms of complex financial instruments called swaps.

The measure is laced with trade-offs. Democrats won budget increases for the Securities and Exchange Commission and the Commodity Futures Trading Commission; Republicans won a big cut to the Internal Revenue Service budget and a smaller cut to the Environmental Protection Agency.

Democrats blocked the most ambitious attempts by Republicans to thwart Obama administration regulations on greenhouse gas emissions blamed for global warming, and on clean water; Republicans won a new concession exempting many agricultural projects from clean water rules and an old one boosting exports of coal mine equipment.

The compromise spending bill would permit virtually the entire government to operate normally through the Sept. 30, 2015, end of the fiscal year, with the exception of the Homeland Security Department.

Fox News' Chad Pergram and The Associated Press contributed to this report.